Conventionally, there has been known such a structure that comprises a concentrically waved metal sheet having opposite surfaces on which expanded-graphite sheets are layered and bonded as a sealing member. This conventional structure employs the concentrically waved metal sheet instead of a core member which comprised a flat metal plate or a metal plate that has opposite surfaces each provided with discontinuous concave and convex portions or worked into a structure having concave and convex portions like saw-teeth, so as to remarkably enhance the compression efficiency, which was extremely small before, and secure so high a deformation-following ability (following ability with respect to the accuracy of the flange-surface) that it can be used for the glass-lining flange distorted or waved. Further, a sheet made of expanded graphite has been utilized for the sealing member as it was done so conventionally. The opposite surfaces in contact with the flange are each formed from a sheet made of expanded graphite, which has a high fluidity and assures an excellent compatibility with the flange.
An explanation is given for a mechanism according to which such a conventional structure exerts the sealing function, with reference to the compression-deformation process of the conventional structure shown in FIG. 8(A) to FIG. 8(C). In these Figures, numeral 1 indicates a concentrically waved metal sheet that constitutes a core member. Numerals 2 and 3 designate expanded-graphite sheets layered and bonded on the opposite surfaces of the metal sheet 1. Numerals 5 and 6 each indicates a flange of a machine or an instrument and a piping. Alphabetical letter (T) means a thickness of the gasket.
In a free state shown in FIG. 8(A) before fastening, the metal sheet 1 has an original wave-pitch (P) and wave-height (T 1/2) (height (T1) of a mountain-like portion) and each of the expanded-graphite sheets 2 and 3 has an original thickness (T2) uniform in its entirety. There are formed vacant gaps 4 between valley portions 1a of the metal sheet 1 and the expanded-graphite sheets 2 and 3.
In a fastened state shown in FIG. 8(B) where a low fastening-load is applied, the metal sheet 1 is compressed between the mutually opposing flanges 5 and 6 in a direction of the thickness while it is decreasing its wave-height (T 1/2) but increasing its wave-pitch (P), so that while making compression-deformation in the thickness-direction, it also performs extension-deformation in a direction of the plane (radially inwards and outward). Although each of the expanded-graphite sheets 2 and 3 is compressed in the thickness-direction at the mountain-like portions 1b of the metal sheet 1, it is not compressed at the valley portions 1a of the metal sheet 1. Therefore, the conventional structure ensures a high fastening surface-pressure locally at the mountain-like portion 1b of the metal sheet 1 to exert a sealing-property. At this time, the expanded-graphite sheets 2 and 3 deform following the position-variation of the mountain-like portions 1b of the metal sheet 1 owing to their high fluidity. Then, without being broken, they are decreasing the thickness T2 at the mountain-like portions 1b of the metal sheet 1 while they are increasing it at the valley portions 1a of the metal sheet 1 to result in partly filling the vacant gaps 4.
In another fastened state shown in FIG. 8(C) where a high fastening-load is applied, the metal sheet 1 is more compressed between the mutually opposing flanges 5 and 6 in the direction of the thickness while it is further decreasing its wave-height (T 1/2) but increasing its wave-pitch (P), so that while making further compression-deformation in the thickness-direction up to a state close to a flat plate, it also performs further extension-deformation in the direction of the plane. At this time, the expanded-graphite sheets 2 and 3 deform following the position-variation of every mountain-like portion 1b of the metal sheet 1 owing to their high fluidity. Then, without being broken, they are further decreasing the thickness T2 at the mountain-like portions 1b of the metal-sheet 1 while they are more increasing it at the valley portions 1a of the metal sheet 1 with the result of ultimately filling the whole vacant gaps 4 completely. Thus the conventional structure exerts a stable sealing-property because the expanded-graphite sheets 2 and 3 are compressed also at the valley-portions 1a of the metal sheet in the thickness direction and secure the fastening surface-pressure over the entire surfaces of the mountain-like portions 1b and the valley portions 1a of the metal sheet 1.
The conventional structure such that the concentrically waved metal sheet has opposite surfaces with the expanded-graphite sheets layered and bonded thereon has been adopted for the gasket described, for example, in Patent Literature 1 and therefore it is publicly known.    Patent Literature: Utility Model Application Laid-Open No. 5-92574